Recuperare colonne usando IRow::GetColumns (o IRow::Open) e ISequentialStream
Si applica a: 
SQL Server Database SQL di Azure Istanza gestita di SQL di Azure Azure Synapse Analytics Piattaforma di strumenti analitici (PDW)
I dati di grandi dimensioni possono essere associati o recuperati utilizzando l'interfaccia ISequentialStream. Per le colonne associate, il flag di stato DBSTATUS_S_TRUNCATED indica che i dati sono troncati.
Importante
Se possibile, usare l'autenticazione di Windows. Se non è disponibile, agli utenti verrà richiesto di immettere le credenziali in fase di esecuzione. Evitare di archiviare le credenziali in un file. Se è necessario rendere persistenti le credenziali, è consigliabile crittografarle usando l'API di crittografia Win32.
Per recuperare le colonne mediante IRow::GetColumns (o IRow::Open) e ISequentialStream
Stabilire una connessione all'origine dati.
Eseguire il comando (nell'esempio seguente viene chiamato ICommandExecute::Execute() con IID_IRow).
Recuperare i dati delle colonne usando IRow::Open() o IRow::GetColumns().
È possibile usare IRow::Open() per aprire un'interfaccia ISequentialStream nella riga. Specificare DBGUID_STREAM per indicare che la colonna contiene un flusso di dati binari. Sarà quindi possibile utilizzare IStream o ISequentialStream per leggere i dati dalla colonna.
Se viene utilizzato IRow::GetColumns(), l'elemento pData della struttura DBCOLUMNACCESS viene impostato in modo da puntare a un oggetto del flusso.
Usare ripetutamente ISequentialStream::Read per leggere il numero specificato di byte nel buffer del consumer.
Esempio
In questo esempio viene illustrato come recuperare una singola riga mediante IRow. In questo esempio viene recuperata una colonna per volta dalla riga. In questo esempio viene illustrato l'utilizzo di IRow::Open() e di IRow::GetColumns(). Per leggere i dati della colonna, nell'esempio viene utilizzato un oggetto ISequentialStream::Read.
Gli esempi di codice Transact-SQL in questo articolo utilizzano il database campione AdventureWorks2022 o AdventureWorksDW2022, che è possibile scaricare dalla home page di Esempi di Microsoft SQL Server e progetti collettivi.
Il primo elenco codici (Transact-SQL) consente di creare una tabella usata dall'esempio.
Compilare il secondo listato di codice (C++) con ole32.lib oleaut32.lib ed eseguirlo. In questa applicazione viene eseguita la connessione all'istanza predefinita di SQL Server nel computer in uso. In alcuni sistemi operativi Windows sarà necessario modificare (local) o (localhost) impostando il valore sul nome dell'istanza di SQL Server. Per connettersi a un'istanza denominata, modificare il stringa di connessione da L"(local)" a L"(local)\name", dove name è l'istanza denominata. Per impostazione predefinita, SQL Server Express viene installato in un'istanza denominata. Verificare che nella variabile di ambiente INCLUDE sia presente la directory che contiene msoledbsql.h.
Il terzo elenco codici (Transact-SQL) consente di eliminare la tabella usata dall'esempio.
USE AdventureWorks2022;
GO
IF EXISTS (SELECT name FROM sysobjects WHERE name = 'MyTable')
DROP TABLE MyTable
GO
CREATE TABLE MyTable
(
col1 int,
col2 varchar(50),
col3 char(50),
col4 datetime,
col5 float,
col6 money,
col7 sql_variant,
col8 binary(50),
col9 text,
col10 image
)
GO
-- Enter data
INSERT INTO MyTable
values
(
10,
'abcdefghijklmnopqrstuvwxyz',
'ABCDEFGHIJKLMNOPQRSTUVWXYZ',
'11/1/1999 11:52 AM',
3.14,
99.95,
CONVERT(nchar(50), N'AbCdEfGhIjKlMnOpQrStUvWxYz'),
0x123456789,
REPLICATE('AAAAABBBBB', 500),
REPLICATE(0x123456789, 500)
)
GO
// compile with: ole32.lib oleaut32.lib
#define DBINITCONSTANTS
#define INITGUID
#define OLEDBVER 0x0250 // to include correct interfaces
#include <stdio.h>
#include <windows.h>
#include <iostream>
#include <oledb.h>
#include <msoledbsql.h>
using namespace std;
const int kMaxBuff = 50;
int InitializeAndEstablishConnection();
HRESULT GetColumnSize(IRow* pUnkRow, ULONG iCol);
ULONG PrintData(ULONG iCols, ULONG iStart, DBCOLUMNINFO* prgInfo, DBCOLUMNACCESS* prgColumns);
HRESULT GetColumns(IRow* pUnkRow, ULONG iStart, ULONG iEnd);
HRESULT GetSequentialColumn(IRow* pUnkRow, ULONG iCol, BOOL fOpen = TRUE);
IDBInitialize* pIDBInitialize = NULL;
IDBProperties* pIDBProperties = NULL;
IDBCreateSession* pIDBCreateSession = NULL;
IDBCreateCommand* pIDBCreateCommand = NULL;
ICommandText* pICommandText = NULL;
IRow* pIRow = NULL;
DBCOLUMNINFO* pDBColumnInfo = NULL;
IAccessor* pIAccessor = NULL;
DBPROP InitProperties[4];
DBPROPSET rgInitPropSet[1];
ULONG i, j;
HRESULT hresult;
DBROWCOUNT cNumRows = 0;
ULONG lNumCols;
WCHAR* pStringsBuffer;
DBBINDING* pBindings;
ULONG ConsumerBufColOffset = 0;
HACCESSOR hAccessor;
ULONG lNumRowsRetrieved;
HROW hRows[10];
HROW* pRows = &hRows[0];
int main() {
ULONG iidx = 0;
WCHAR* wCmdString = OLESTR("SELECT * FROM MyTable ");
// Call a function to initialize and establish connection.
if (InitializeAndEstablishConnection() == -1) {
cout << "Failed to initialize and establish connection.\n";
// Insert your code for cleanup and error handling
return -1;
}
// Create a session object.
if (FAILED(pIDBInitialize->QueryInterface( IID_IDBCreateSession, (void**) &pIDBCreateSession))) {
cout << "Failed to obtain IDBCreateSession interface.\n";
// Insert your code for cleanup and error handling
return -1;
}
if (FAILED(pIDBCreateSession->CreateSession( NULL,
IID_IDBCreateCommand,
(IUnknown**) &pIDBCreateCommand))) {
cout << "pIDBCreateSession->CreateSession failed.\n";
// Insert your code for cleanup and error handling
return -1;
}
// Access the ICommandText interface.
if (FAILED(pIDBCreateCommand->CreateCommand( NULL, IID_ICommandText, (IUnknown**) &pICommandText))) {
cout << "Failed to access ICommand interface.\n";
// Insert your code for cleanup and error handling
return -1;
}
// Use SetCommandText() to specify the command text.
if (FAILED(pICommandText->SetCommandText(DBGUID_DBSQL, wCmdString))) {
cout << "Failed to set command text.\n";
// Insert your code for cleanup and error handling
return -1;
}
// Execute the command.
if (FAILED(hresult = pICommandText->Execute( NULL, IID_IRow, NULL, &cNumRows, (IUnknown **) &pIRow))) {
cout << "Failed to execute command.\n";
// Insert your code for cleanup and error handling
return -1;
}
DBORDINAL cColumns = 0;
DBCOLUMNINFO* prgInfo = 0;
OLECHAR* pColNames;
// Get column count
HRESULT hr;
IColumnsInfo* pIColumnsInfo;
hr = pIRow->QueryInterface(IID_IColumnsInfo, (void**) &pIColumnsInfo);
if (FAILED(hr))
goto CLEANUP;
hr = pIColumnsInfo->GetColumnInfo(&cColumns, &prgInfo, &pColNames);
if (FAILED(hr))
goto CLEANUP;
// Get columns (one at a time) using ISequentialStream and Open
// 3rd parameter is by default TRUE indicating use of ISequentialStream and Open.
DBCOLUMNINFO* colInfo;
for ( iidx = 1 ; iidx <= cColumns ; iidx++ ) {
colInfo = (DBCOLUMNINFO*)(prgInfo + (iidx - 1));
if (colInfo->dwFlags & DBCOLUMNFLAGS_ISLONG)
hresult = GetSequentialColumn(pIRow, iidx);
else
hresult = GetColumns(pIRow, iidx, iidx);
}
// Release the Row object.
pIRow->Release();
// Execute the command again.
if (FAILED(hresult = pICommandText->Execute(NULL,
IID_IRow,
NULL,
&cNumRows,
(IUnknown **) &pIRow))) {
cout << "Failed to execute command.\n";
// Insert your code for cleanup and error handling
return -1;
}
// Now get columns (one at a time) using ISequentialStream and GetColumns.
// The 3rd parameter is by default TRUE indicating use of ISequentialStream
// and GetColumns
for ( iidx = 1 ; iidx <= cColumns ; iidx++ ) {
colInfo = (DBCOLUMNINFO*)(prgInfo + (iidx - 1));
if (colInfo->dwFlags & DBCOLUMNFLAGS_ISLONG)
hresult = GetSequentialColumn(pIRow, iidx, FALSE);
else
hresult = GetColumns(pIRow, iidx, iidx);
}
CLEANUP:
// Release memory.
pICommandText->Release();
pIDBCreateCommand->Release();
pIDBCreateSession->Release();
if (FAILED(pIDBInitialize->Uninitialize())) {
// Uninitialize not required, but it fails if an interface has not been released, can be used for debugging.
// cout << "Problem uninitializing.\n";
}
pIDBInitialize->Release();
CoTaskMemFree(prgInfo);
CoTaskMemFree(pColNames);
if( pIColumnsInfo )
pIColumnsInfo->Release();
// Release the COM library.
CoUninitialize();
}
HRESULT GetSequentialColumn(IRow* pUnkRow, ULONG iCol, BOOL fOpen) {
HRESULT hr = NOERROR;
ULONG cbRead = 0;
ULONG cbTotal = 0;
DBORDINAL cColumns = 0;
ULONG cReads = 0;
ISequentialStream* pIStream = NULL;
WCHAR* pBuffer[kMaxBuff]; // 50 chars read by ISequentialStream::Read()
DBCOLUMNINFO* prgInfo = 0;
OLECHAR* pColNames = 0;
IColumnsInfo* pIColumnsInfo;
DBID columnid;
DBCOLUMNACCESS column;
wprintf(L"[RETRIEVING COLUMN %d SEQUENTIALLY]\n", iCol);
// Get column information (basically get column id).
hr = pUnkRow->QueryInterface(IID_IColumnsInfo, (void**) &pIColumnsInfo);
if (FAILED(hr))
goto CLEANUP;
hr = pIColumnsInfo->GetColumnInfo(&cColumns, &prgInfo, &pColNames);
if (FAILED(hr))
goto CLEANUP;
// Get Column ID.
columnid = (prgInfo + (iCol - 1))->columnid;
if (fOpen) { // Get columns using ISequentialStream and IRow::Open
wprintf(L"[RETRIEVING COLUMNS USING ");
wprintf(L" ISequentialSteam and Open]\n");
// Open sequential stream.
hr = pUnkRow->Open(NULL,
&columnid,
DBGUID_STREAM,
0,
IID_ISequentialStream,
(LPUNKNOWN *)&pIStream);
if (FAILED(hr)) {
wprintf(L"Unable to get ISequentialStream interface.\n");
goto CLEANUP;
}
}
else { // Get Columns using IRow::GetColumns and ISequentialStream.
wprintf(L"[RETRIEVING COLUMNS USING ");
wprintf(L" GetColumns and ISequentialStream]\n");
IUnknown* pUnkStream = NULL;
ZeroMemory(&column, sizeof(column));
column.columnid = prgInfo[iCol - 1].columnid;
column.wType = DBTYPE_IUNKNOWN;
column.pData = (LPVOID*) &pUnkStream;
hr = pUnkRow->GetColumns(1, &column);
if (FAILED(hr)) {
wprintf(L"Error executing IRow::GetColumns.\n");
goto CLEANUP;
}
hr = pUnkStream->QueryInterface(IID_ISequentialStream, (LPVOID*) &pIStream);
if (FAILED(hr)) {
wprintf(L"Unable to get ISequentialStream interface ");
wprintf(L"via IRow::GetColumns.\n");
goto CLEANUP;
}
pUnkStream->Release();
}
ZeroMemory(pBuffer, kMaxBuff * sizeof(WCHAR));
// Read 50 chars at a time until no more data.
do {
hr = pIStream->Read(pBuffer, kMaxBuff, &cbRead);
if (FAILED(hr)) {
wprintf(L"Error reading data.\n");
goto CLEANUP;
}
cbTotal = cbTotal + cbRead;
// Print the data
wprintf(L"READ #%d: %-*S\n", ++cReads, kMaxBuff, pBuffer);
} while(cbRead > 0);
wprintf(L"[READ %d bytes for column %d.\n", cbTotal, iCol);
CLEANUP:
if (pIColumnsInfo)
pIColumnsInfo->Release();
CoTaskMemFree(prgInfo);
CoTaskMemFree(pColNames);
if (pIStream)
pIStream->Release();
return hr;
}
BOOL InitColumn(DBCOLUMNACCESS* pCol, DBCOLUMNINFO* pInfo) {
// If maximum possible length of a value column is very large (text or image
// column is read) limit that size to 512 bytes (for illustration purposes).
ULONG ulSize;
if (pInfo->wType == DBTYPE_WSTR || pInfo->wType == DBTYPE_STR)
ulSize = (pInfo->ulColumnSize < 0x7fffffff) ? pInfo->ulColumnSize : 512;
else
ulSize = 128; //default buffer to handle conversion to text of non-string data types
// Verify data buffer is large enough.
if (pCol->cbMaxLen < (ulSize + 1)) {
if (pCol->pData) {
delete [] pCol->pData;
pCol->pData = NULL;
}
// Allocate data buffer
void * p = pCol->pData = new WCHAR[ulSize + 1];
if (!(p /*pCol->pData = new WCHAR[ulSize + 1]*/ ))
return FALSE;
// set the max length of caller-initialized memory.
pCol->cbMaxLen = sizeof(WCHAR) * (ulSize + 1);
// In the above 2 steps, pData is pointing to memory (it is not NULL) and
// cbMaxLen has a value (not 0), so next call to IRow->GetData()
// will read the data from the column.
}
// Clear memory buffer
ZeroMemory((void*) pCol->pData, pCol->cbMaxLen);
// Set properties.
//pCol->wType = DBTYPE_WSTR;
pCol->wType = DBTYPE_WSTR;
pCol->columnid = pInfo->columnid;
pCol->cbDataLen = 0;
pCol->dwStatus = 0;
pCol->dwReserved = 0;
pCol->bPrecision = 0;
pCol->bScale = 0;
return TRUE;
}
HRESULT GetColumns(IRow* pUnkRow, ULONG iStart, ULONG iEnd) {
// Start and end are same. Thus, get only one column.
HRESULT hr = E_FAIL;
ULONG iidx; // loop counter
DBORDINAL cColumns; // Count of columns
ULONG cUserCols; // Count of user columns
DBCOLUMNINFO* prgInfo; // Column of info. array
OLECHAR* pColNames; // Array of column names
DBCOLUMNACCESS* prgColumns; // Ptr to column access structures array
DBCOLUMNINFO* pCurrInfo;
DBCOLUMNACCESS* pCurrCol;
IColumnsInfo* pIColumnsInfo = NULL;
// Initialize
cColumns = 0;
prgInfo = NULL;
pColNames = NULL;
prgColumns = NULL;
printf("Retrieving data with GetColumns\n");
// Get column info to build column access array
hr = pUnkRow->QueryInterface(IID_IColumnsInfo, (void**)&pIColumnsInfo);
if (FAILED(hr))
goto CLEANUP;
hr = pIColumnsInfo->GetColumnInfo(&cColumns, &prgInfo, &pColNames);
if (FAILED(hr))
goto CLEANUP;
// Determine no. of columns to retrieve. Since iEnd and iStart is same,
// this is redundent step. cUserCols will always be 1.
cUserCols = iEnd - iStart + 1;
// Walk list of columns and setup a DBCOLUMNACCESS structure
DBCOLUMNACCESS * x = (prgColumns = new DBCOLUMNACCESS[cUserCols]);
if (!(x /*prgColumns = new DBCOLUMNACCESS[cUserCols]*/ )) { // cUserCols is only 1
hr = E_FAIL;
goto CLEANUP;
}
ZeroMemory((void*) prgColumns, sizeof(DBCOLUMNACCESS) * cUserCols);
for ( iidx = 0 ; iidx < cUserCols ; iidx++ ) {
pCurrInfo = prgInfo + iidx + iStart - 1;
pCurrCol = prgColumns + iidx;
// Here the values of pData and cbMaxLen elements of DBCOLUMNACCESS
// elements is set. Thus IRow->GetColumns() will return actual data.
if ( InitColumn(pCurrCol, pCurrInfo) == FALSE )
goto CLEANUP;
}
hr = pUnkRow->GetColumns(cUserCols, prgColumns); // cUserCols = 1
if (FAILED(hr))
printf("Error occurred\n");
// Show data.
PrintData(cUserCols, iStart, prgInfo, prgColumns);
CLEANUP:
if (pIColumnsInfo)
pIColumnsInfo->Release();
if (prgColumns)
delete [] prgColumns;
return hr;
}
// This function returns the actual width of the data in the column (not the
// columnwidth in DBCOLUMNFO structure which is the width of the column)
HRESULT GetColumnSize(IRow* pUnkRow, ULONG iCol) {
HRESULT hr = NOERROR;
DBORDINAL cColumns = 0; // Count the columns
DBCOLUMNINFO* prgInfo; // Column info array
OLECHAR* pColNames;
DBCOLUMNACCESS column;
DBCOLUMNINFO* pCurrInfo;
IColumnsInfo* pIColumnsInfo = NULL;
// Initialize
prgInfo = NULL;
pColNames = NULL;
printf("Checking column size\n");
// Get column info to build column access array
hr = pUnkRow->QueryInterface(IID_IColumnsInfo, (void**) &pIColumnsInfo);
if (FAILED(hr))
goto CLEANUP;
hr = pIColumnsInfo->GetColumnInfo(&cColumns, &prgInfo, &pColNames);
if (FAILED(hr))
goto CLEANUP;
printf("Value of cColumns is %d\n", cColumns);
// Setup a DBCOLUMNACCESS structure: pData is set to NULL and cbMaxLen is set
// to 0. Thus IRow->GetColumns() returns only the actual column length in
// cbDataLen member of DBCOLUMNACCESS structure.In this case you can call
// IRow->GetColumns() again for the same column to retrieve actual data in the second call.
ZeroMemory((void*) &column, sizeof(DBCOLUMNACCESS));
column.pData = NULL;
pCurrInfo = prgInfo + iCol - 1;
// Get column id in DBCOLUMNACCESS structure. It is then used in GetColumn().
column.columnid = pCurrInfo->columnid;
printf("column.columnid value is %d\n", column.columnid);
// We know which column to get. The column.columnid gives the column number.
hr = pUnkRow->GetColumns(1, &column);
if (FAILED(hr))
printf("Errors occurred\n");
// Show data
PrintData(1, iCol, prgInfo, &column);
CLEANUP:
if (pIColumnsInfo)
pIColumnsInfo->Release();
return hr;
}
BOOL GetStatus(DWORD dwStatus, WCHAR* pwszStatus) {
switch (dwStatus) {
case DBSTATUS_S_OK:
wcscpy_s(pwszStatus, 255, L"DBSTATUS_S_OK");
break;
case DBSTATUS_E_CANTCONVERTVALUE:
wcscpy_s(pwszStatus, 255, L"DBSTATUS_E_CANTCONVERTVALUE");
break;
case DBSTATUS_S_ISNULL:
wcscpy_s(pwszStatus, 255, L"DBSTATUS_S_ISNULL");
break;
case DBSTATUS_E_UNAVAILABLE:
wcscpy_s(pwszStatus, 255, L"DBSTATUS_E_UNAVAILABLE");
break;
case DBSTATUS_S_TRUNCATED:
wcscpy_s(pwszStatus, 255, L"DBSTATUS_S_TRUNCATED");
break;
default:
swprintf_s(pwszStatus, sizeof(pwszStatus), L"OTHER STATUS VALUE: %d", dwStatus);
}
return TRUE;
}
ULONG PrintData(ULONG iCols,
ULONG iStart,
DBCOLUMNINFO* prgInfo,
DBCOLUMNACCESS* prgColumns) {
WCHAR wszStatus[255];
DBCOLUMNINFO* pCurrInfo;
DBCOLUMNACCESS* pCurrCol;
ULONG iidx = 0; // Loop counter
printf("%-3s %-20s %-21s %-9s %-9s %-50s\n", "No.", "Name", "Status", "Length", "Max", "Data");
for ( iidx = 0 ; iidx < iCols ; iidx++ ) {
pCurrInfo = prgInfo + iidx + iStart - 1;
pCurrCol = prgColumns + iidx;
GetStatus(pCurrCol->dwStatus, wszStatus);
// was the data successfully retrieved?
wprintf(L"%-3d %-20s %-21s %-9d %-9d %-50s\n", iStart + iidx,
pCurrInfo->pwszName,
wszStatus,
pCurrCol->cbDataLen,
pCurrCol->cbMaxLen,
pCurrCol->dwStatus == DBSTATUS_S_ISNULL ? L"(NULL)" : (WCHAR*) pCurrCol->pData);
}
wprintf(L"\n");
return iidx;
}
int InitializeAndEstablishConnection() {
// Initialize the COM library.
CoInitialize(NULL);
// Obtain access to the MSOLEDBSQL provider.
hresult = CoCreateInstance(CLSID_MSOLEDBSQL,
NULL,
CLSCTX_INPROC_SERVER,
IID_IDBInitialize,
(void **) &pIDBInitialize);
if(FAILED(hresult)) {
printf("Failed to get IDBInitialize interface.\n");
// Insert your code for cleanup and error handling
return -1;
}
// Initialize the property values needed to establish the connection.
for (i = 0 ; i < 4 ; i++ )
VariantInit(&InitProperties[i].vValue);
// Server name.
InitProperties[0].dwPropertyID = DBPROP_INIT_DATASOURCE;
InitProperties[0].vValue.vt = VT_BSTR;
InitProperties[0].vValue.bstrVal = SysAllocString(L"(local)");
InitProperties[0].dwOptions = DBPROPOPTIONS_REQUIRED;
InitProperties[0].colid = DB_NULLID;
// Database.
InitProperties[1].dwPropertyID = DBPROP_INIT_CATALOG;
InitProperties[1].vValue.vt = VT_BSTR;
InitProperties[1].vValue.bstrVal= SysAllocString(L"AdventureWorks");
InitProperties[1].dwOptions = DBPROPOPTIONS_REQUIRED;
InitProperties[1].colid = DB_NULLID;
// connection
InitProperties[2].dwPropertyID = DBPROP_AUTH_INTEGRATED;
InitProperties[2].vValue.vt = VT_BSTR;
InitProperties[2].vValue.bstrVal= SysAllocString(L"SSPI");
InitProperties[2].dwOptions = DBPROPOPTIONS_REQUIRED;
InitProperties[2].colid = DB_NULLID;
// Now that the properties are set, construct the DBPROPSET structure
// (rgInitPropSet). The DBPROPSET structure is used to pass an array
// of DBPROP structures (InitProperties) to the SetProperties method.
rgInitPropSet[0].guidPropertySet = DBPROPSET_DBINIT;
rgInitPropSet[0].cProperties = 4;
rgInitPropSet[0].rgProperties = InitProperties;
// Set initialization properties.
hresult = pIDBInitialize->QueryInterface(IID_IDBProperties, (void **)&pIDBProperties);
if (FAILED(hresult)) {
cout << "Failed to get IDBProperties interface.\n";
// Insert your code for cleanup and error handling
return -1;
}
hresult = pIDBProperties->SetProperties(1, rgInitPropSet);
if (FAILED(hresult)) {
cout << "Failed to set initialization properties.\n";
// Insert your code for cleanup and error handling
return -1;
}
pIDBProperties->Release();
// Now establish the connection to the data source.
if (FAILED(pIDBInitialize->Initialize())) {
cout << "Problem establishing connection to the data source.\n";
// Insert your code for cleanup and error handling
return -1;
}
return 0;
}
USE AdventureWorks2022;
GO
IF EXISTS (SELECT name FROM sysobjects WHERE name = 'MyTable')
DROP TABLE MyTable
GO